This invention relates to a power brake unit for automotive vehicles including a brake valve which is adapted to meter pressure transmitting fluid from an auxiliary energy source into a booster chamber which is isolated from an intermediate pressure chamber by an axially displaceable booster piston arrangement, the other side of the intermediate pressure chamber being bounded by a master cylinder piston or an intermediate piston coupled thereto, with a hydraulic transmission ratio which is effective between the brake pedal and the master cylinder piston being disconnectible dependent on the pressure of the auxiliary energy source.
A brake booster of this type is described in German Patent DE-OS 2,460,529.
By selecting a high pressure in the auxiliary energy source and suitably dimensioning the effective surfaces of the booster piston arrangement, it is possible to achieve a high brake boosting action with power brake units of this type without entailing great expense. The power brake unit is therefore particularly suitable for use in heavy duty vehicles. The disconnectible hydraulic transmission ratio serves to modify the transmission ratio between brake pedal and master cylinder piston if the auxiliary energy fails because of a defect. This is necessary to enable a brake pressure as high as possible to be built up with a relatively low pedal force and without the assistance of the auxiliary energy, utilizing the maximum permissible pedal travel. With the transmission ratio constant in a brake system designed for operation without auxiliary energy, the additional action of the auxiliary energy would cause the pedal to be stepped down to the floorboard of the vehicle so that a further pressure build-up would be prevented as a result of the blocked pedal travel.
The disconnectible transmission ratio is embodied in the power brake unit of the above cited German Patent by arranging for the booster piston to have its effective surface adjacent the master cylinder piston in an intermediate pressure chamber, this surface being of a cross section larger than the effective surface of the master cylinder piston facing it, and by arranging for the intermediate pressure chamber to be connectible with the working chamber in from of the master cylinder piston through a valve arrangement.
Disadvantages of this power brake unit are that the valve arrangement necessary for connecting and disconnecting the hydraulic transmission ratio necessitates substantial additional costs and the valve arrangement may be the cause of a failure of the power brake unit by remaining in a permanently open or closed position because of a defect. If such a defect occurs, sufficient deceleration of the vehicle is no longer possible using the pedal force alone when the auxiliary energy has failed, or, in the presence of the auxiliary energy, the brake pedal travels down to the floorboard, thereby limiting the further build-up of pressure.